WeakMap with Symbol key used in "global" features store is incompatible with FireFox #57
Description
Due to this bug, FireFox does not support using non-registered symbols as WeakMap keys.
This means use of this store is currently ineffective on FireFox, thus making it unavailable for production use.
Moving to use an object held in a closure is probably the simplest remedy for this. However, It may be worth considering aligning the globalFeatureStoreFactory to use a runtime-wide global symbol, as per change made to the nodeRequestScopedFeaturesStoreFactory in #51 - rather than its current behaviour which allows multiple stores for the same toggle type, which is now a required field due to this update.
A need for multiple stores for the same toggle type is unclear, so perhaps can be aligned to a truly "global" store as per the node factory?